Meters used on motorcycles include digital displays of running time (hours), engine temperature and other information. Since the motorcycles may be of the trail-type for traversing rough terrain, the meters must be robust to withstand vibration and shock. The meters must also be well sealed to prevent penetration of fine dust and liquid into the meter.
The meters of interest here include, among other components, a digital LCD that is backlit by light that is provided by a light guide. In this case, the light guide is a transparent glass-like solid through which light propagates from one end of the guide to another. The light guide is coupled to the display and transmits light from a source, such as one or more light emitting diodes (LEDs) carried on a nearby printed circuit board, to the back side of the display for illuminating or “back lighting” the display for viewing through its transparent front or display side. The back lighting is especially helpful for viewing under low ambient light conditions.
The meter components are housed in a casing that is mounted to the motorcycle. A cable is often included to deliver information from a remote sensor to the meter. An opening in the casing permits viewing of the front of the display. To enhance the robustness of the meter, the assembly process may include a liquid epoxy resin that is injected into the casing interior after the display, light guide, and associated circuit board are in place. The epoxy cures to encapsulate the components against penetration by damaging liquid or dust. The epoxy also serves a dual purpose of anchoring the components in the casing interior for securing the components in place, and for damping vibrations that are received by the meter casing to thus protect the interior components.
One problem with the epoxy encapsulation technique just mentioned is ensuring that the epoxy does not come into direct contact with the light guide, which may be, for example, formed of optically-clear acrylic. Such contact with the exterior of the light guide will change the refraction characteristics of the outer surface of the guide so that light inside the guide (and propagating from the source to back light the display) will be absorbed by the epoxy. This absorption reduces the illumination of the display to unacceptably low levels. The absorbed, lost light is sometimes referred to as light that has “leaked” from the light guide.
One prior solution to the foregoing problem is to place a reflective tape around the exterior of the surfaces that are intended to reflect light along the light guide. This approach, however, is time consuming and requires a labor-intensive assembly process. Moreover, given the complex shape of some light guides it is difficult to effectively apply the tape to all of the surface portions that may be exposed to contact with the later applied epoxy.
The present invention is directed to a solution to the foregoing problems and provides, among other things, an innovative way of assembling a display device to which is coupled a light guide that has smooth surfaces for light-transmission and at least one surface that is configured for light scattering and internal reflection. The method includes depositing on that surface a reflective coating, and installing the display device with coupled light guide in an interior of a meter housing or casing. At least a portion of the casing interior is filled with an epoxy for sealing the display device with coupled light guide against water and dust penetration. All surfaces of the light guide are protected against direct contact with the epoxy or with any contaminant. This approach greatly simplifies the assembly process and yields a robust display without light leaking from the light guide.
Other advantages and features of the present invention will become clear upon study of the following portion of this specification and drawings.